Foams made from water-absorbing, basic polymers, method for the production and utilization thereof

ABSTRACT

Foams composed of water-absorbing basic polymers, obtainable by (I) foaming a crosslinkable aqueous mixture including (a) at least one basic polymer whose basic groups have optionally been neutralized, (b) at least one crosslinker, (c) at least one surfactant, (d) optionally at least one solubilizer, (e) optionally thickeners, foam stabilizers, fillers, fibers and/or cell nucleators, and (f) optionally particulate water-absorbing acidic polymers, by dissolving a gas which is inert toward free radicals in the crosslinkable aqueous mixture under a pressure from 2 to 400 bar and subsequently decompressing the crosslinkable aqueous mixture to atmospheric or by dispersing fine bubbles of a gas which is inert toward free radicals, and (II) crosslinking the foamed mixture to form a hydrogel foam and if applicable adjusting the water content of the polymer foam to 1-60% by weight. Preparation of the foams by application of the abovementioned measures (I) and (II) and use of the thus obtainable foams in hygiene articles to absorb body fluids, in dressing material to cover wounds, as a sealing material, as a packaging material, as a soil improver, as a soil substitute, to dewater sludges, to absorb aqueous acidic wastes, to thicken waterborne paints or coatings in the course of disposing of residual quantities thereof, to dewater water-containing oils or hydrocarbons or as a material for filters in ventilation systems.

This invention relates to foams composed of water-absorbing basicpolymers, processes for their preparation and their use in hygienearticles.

Water-absorbing, predominantly open-celled foams based on crosslinkedacid-functional monomers are known, cf. EP-B-0 858 478, WO-A-99/44648and WO-A-00/52087. They are prepared for example by foaming apolymerizable aqueous mixture containing at least 50 mol % neutralizedacid-functional monoethylenically unsaturated monomers, crosslinkers andat least one surfactant and subsequently polymerizing the foamedmixture. The foaming of the polymerizable mixture can be effected forexample by dispersing fine bubbles of a gas which is inert toward freeradicals or by dissolving such a gas under elevated pressure in thepolymerizable mixture and decompressing the mixture. The water contentof the foams is adjusted to 1-60% by weight for example. The foams canoptionally be subjected to surface postcrosslinking by spraying acrosslinker onto the foamed material or immersing the foam therein andheating the crosslinker-laden foam to a relatively high temperature. Thefoams are used for example in hygiene articles to acquire, distributeand store body fluids.

WO-A-97/31600 discloses an absorber element for use in hygiene orsanitary articles wherein a plurality of elements of a superabsorbentfoam are arranged on a support in a grid pattern at such distances thatthe elements in the swollen state touch at their peripheries. Forexample, a monomer foam can be applied to the support in the desiredgrid pattern and then polymerized or separately prepared foam elementscan be fixed on the support in the desired grid pattern by chemical orphysical means. However, the permeability of the superabsorbent foams isstill in need of improvement.

U.S. Pat. No. 5,981,689 and U.S. Pat. No. 6,121,409 disclosewater-absorbing materials based on polyvinylamine gels. These materialsconsist essentially of a mixture of particulate polymers composed of alightly crosslinked basic polymer, which has optionally been surfacepostcrosslinked, and an acidic polymer, each polymer being capable ofabsorbing water when in the polyelectrolyte form. Examples of suchabsorbing products are mixtures of particles of lightly crosslinkedpolyvinylamine in the form of the free base with particles of lightlycrosslinked polyacrylic acid in the form of the free acid. As U.S. Pat.No. 5,981,689 also reveals, salts of crosslinked polyvinylamines can beused as water-absorbing polymers.

U.S. Pat. No. 5,962,578 likewise discloses water-absorbing materialscomposed of a mixture of particles of a crosslinked basic polymer andparticles of an acidic water-absorbing polymer. Lightly crosslinkedaddition polymers of dialkylaminoalkyl-(meth)acrylamides are specifiedas a basic polymer. Similarly, lightly crosslinked particulate polymersof polyvinylguanidines can be used as basic polymers in mixture withacidic water-absorbing particulate polymers, cf. U.S. Pat. No.6,087,448.

U.S. Pat. No. 6,222,091 discloses water-absorbing gel particles whereineach particle comprises microdomains of an acidic water-absorbing resinand of a basic water-absorbing resin. The above-describedwater-absorbing materials are used for example in hygiene articles toabsorb body fluids.

WO-A-00/63295 likewise discloses hydrogel-forming particulate mixturesconsisting of a lightly crosslinked basic polymer and a lightlycrosslinked acidic polymer.

It is an object of the present invention to provide water-absorbingarticles having a high absorption capacity, an improved permeability andan improved distribution.

We have found that this object is achieved by predominantly open-celledfoams composed of water-absorbing basic polymers, obtainable by

-   -   (I) foaming a crosslinkable aqueous mixture including        -   (a) at least one basic polymer whose basic groups have            optionally been neutralized,        -   (b) at least one crosslinker,        -   (c) at least one surfactant,        -   (d) optionally at least one solubilizer,        -   (e) optionally thickeners, foam stabilizers, fillers, fibers            and/or cell nucleators, and        -   (f) optionally particulate water-absorbing acidic polymers,        -   by dissolving a gas which is inert toward free radicals in            the crosslinkable aqueous mixture under a pressure from 2 to            400 bar and subsequently decompressing the crosslinkable            aqueous mixture to atmospheric or by dispersing fine bubbles            of a gas which is inert toward free radicals, and    -   (II) crosslinking the foamed mixture to form a hydrogel foam and        if applicable adjusting the water content of the polymer foam to        1-60% by weight.

The present invention further provides a process for producing foamscomposed of water-absorbing basic polymers, which comprises

-   -   (I) foaming a crosslinkable aqueous mixture including        -   (a) at least one basic polymer whose basic groups have            optionally been neutralized,        -   (b) at least one crosslinker,        -   (c) at least one surfactant,        -   (d) optionally at least one solubilizer,        -   (e) optionally thickeners, foam stabilizers, fillers, fibers            and/or cell nucleators, and        -   (f) optionally particulate water-absorbing acidic polymers,        -   by dissolving a gas which is inert toward free radicals in            the crosslinkable aqueous mixture under a pressure from 2 to            400 bar and subsequently decompressing the crosslinkable            aqueous mixture to atmospheric or by dispersing fine bubbles            of a gas which is inert toward free radicals, and    -   (II) crosslinking the foamed mixture to form a hydrogel foam and        if applicable adjusting the water content of the polymer foam to        1-60% by weight.

(a) Basic Polymers

Useful basic polymers include for example polymers containing vinylamineunits, polymers containing vinylguanidine units, polymers containingdialkylaminoalkyl(meth)acrylamide units, polyethyleneimines,ethyleneimine-grafted polyamidoamines and polydiallyldimethylammoniumchlorides.

Polymers containing vinylamine units are known, cf. U.S. Pat. No. 4,421,602, U.S. Pat. No. 5,334,287, EP-A-0 216 387, U.S. Pat. No.5,981,689, WO-A-00/63295 and U.S. Pat. No. 6,121,409. They are preparedby hydrolysis of polymers containing open-chain N-vinylcarboxamideunits. These polymers are obtainable for example by polymerizingN-vinylformamide, N-vinyl-N-methylformamide, N-vinylacetamide,N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide andN-vinylpropionamide. The monomers mentioned can be polymerized eitheralone or together with other monomers.

Useful monoethylenically unsaturated monomers for copolymerization withthe N-vinylcarboxamides include all compounds copolymerizable therewith.Examples thereof are vinyl esters of saturated carboxylic acids of 1 to6 carbon atoms such as vinyl formate, vinyl acetate, vinyl propionateand vinyl butyrate and vinyl ethers such as C₁-C₆-alkyl vinyl ethers,for example methyl vinyl ether or ethyl vinyl ether. Useful comonomersfurther include esters, amides and nitriles of ethylenically unsaturatedC₃-C₆-carboxylic acids, such as for example methyl acrylate, methylmethacrylate, ethyl acrylate and ethyl methacrylate, acrylamide andmethacrylamide and also acrylonitrile and methacrylonitrile.

Useful carboxylic esters are further derived from glycols orpolyalkylene glycols, in either case only one OH group being esterified,for example hydroxyethyl acrylate, hydroxyethyl methacrylate,hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxypropylmethacrylate, hydroxybutyl methacrylate and also acrylic monoesters ofpolyalkylene glycols having a molar mass from 500 to 10 000. Usefulcomonomers further include esters of ethylenically unsaturatedcarboxylic acids with amino alcohols such as for exampledimethylaminoethyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl acrylate, diethylaminoethyl methacrylate,dimethylaminopropyl acrylate, dimethylaminopropyl methacrylate,diethylaminopropyl acrylate, dimethylaminobutyl acrylate anddiethylaminobutyl acrylate. The basic acrylates can be used in the formof the free bases, in the form of their salts with mineral acids such ashydrochloric acid, sulfuric acid or nitric acid, in the form of theirsalts with organic acids such as formic acid, acetic acid, propionicacid or sulfonic acids or in quaternized form. Useful quaternizingagents include for example dimethyl sulfate, diethyl sulfate, methylchloride, ethyl chloride or benzyl chloride.

Useful comonomers further include amides of ethylenically unsaturatedcarboxylic acids such as acrylamide, methacrylamide and alsoN-alkylmonoamides and -diamides of monoethylenically unsaturatedcarboxylic acids having alkyl moieties of 1 to 6 carbon atoms, forexample N-methylacrylamide, N,N-dimethylacrylamide,N-methylmethacrylamide, N-ethylacrylamide, N-propylacrylamide andtert-butylacrylamide and also basic (meth)acrylamides, for exampledimethylaminoethylacrylamide, dimethylaminoethylmethacrylamide,diethylaminoethylacrylamide, diethylaminoethylmethacrylamide,dimethylaminopropylacrylamide, diethylaminopropylacrylamide,dimethylaminopropylmethacrylamide and diethylaminopropylmethacrylamide.

Useful comonomers further include N-vinylpyrrolidone,N-vinylcaprolactam, acrylonitrile, methacrylonitrile, N-vinylimidazoleand also substituted N-vinylimidazoles such as for exampleN-vinyl-2-methylimidazole, N-vinyl-4-methylimidazole,N-vinyl-5-methylimidazole, N-vinyl-2-ethylimidazole andN-vinylimidazolines such as N-vinylimidazoline,N-vinyl-2-methylimidazoline and N-vinyl-2-ethylimidazoline.N-Vinylimidazoles and N-vinylimidazolines are used not only in the formof the free bases but also after neutralization with mineral acids ororganic acids or in quaternized form, in which case the quaternizationis preferably effected using dimethyl sulfate, diethyl sulfate, methylchloride or benzyl chloride. It is further possible to usediallyldialkylammonium halides, for example diallyldimethylammoniumchloride.

The copolymers contain for example

-   -   from 95 to 5 mol % and preferably from 90 to 10 mol % of at        least one N-vinylcarboxamide, and    -   from 5 to 95 mol %, and preferably from 10 to 90 mol % of other        monoethylenically unsaturated monomers copolymerizable therewith        in copolymerized form. The comonomers are preferably free of        acid groups.

To prepare polymers containing vinylamine units, it is preferable tostart from homopolymers of N-vinylformamide or from copolymersobtainable by copolymerizing

-   -   N-vinylformamide with    -   vinyl formate, vinyl acetate, vinyl propionate, acrylonitrile,        N-vinylcaprolactam, N-vinyl urea, N-vinylpyrrolidone or        C₁-C₆-alkyl vinyl ethers        and subsequently hydrolyzing the homo- or copolymers to form        vinylamine units from the copolymerized N-vinylformamide units,        the degree of hydrolysis being for example in the range from 5        to 100 mol % and preferably in the range from 70 to 100 mol %.        The hydrolysis of the above-described polymers is effected        according to known processes by the action of acids, bases or        enzymes. When acids are used as a hydrolyzing agent, the        vinylamine units of the polymers are present as an ammonium        salt, whereas the hydrolysis with bases gives rise to free amino        groups.

The degree of hydrolysis of the homopolymers of the N-vinylcarboxamidesand their copolymers can be in the range from 5 to 100 mol % and ispreferably in the range from 70 to 100 mol %. In most cases, the degreeof hydrolysis of the homo- and copolymers is in the range from 80 to 95mol %. The degree of hydrolysis of the homopolymers is equivalent to thelevel of vinylamine units in the polymers. In the case of copolymerswhich contain vinyl esters in copolymerized form, the hydrolysis of theN-vinylformamide units may be accompanied by a hydrolysis of the estergroups to form vinyl alcohol units. This is particularly the case whenthe hydrolysis of the copolymers is conducted in the presence of aqueoussodium hydroxide solution. Polymerized units of acrylonitrile willlikewise undergo chemical changes in the course of the hydrolysis,producing for example amide groups or carboxyl groups. The homo- andcopolymers containing vinylamine units may contain up to 20 mol % ofamidine units, for example due to a reaction of formic acid with twoadjacent amino groups or due to intramolecular reaction of an aminogroup with an adjacent amide group, for example of copolymerizedN-vinylformamide. The molar masses of the polymers containing vinylamineunits range for example from 500 to 10 million and preferably from 1000to 5 million (determined by light scattering). This molar mass rangecorresponds for example to K values from 5 to 300 and preferably from 10to 250 (determined after H. Fikentscher in 5% aqueous sodium chloridesolution at 25° C. and at a polymer concentration of 0.5% by weight).

The polymers containing vinylamine units are preferably used insalt-free form. Salt-free aqueous solutions of polymers containingvinylamine units are preparable for example from the above-describedsalt-containing polymer solutions by ultrafiltration using suitablemembranes having molecular weight cutoffs at for example from 1000 to500 000 Dalton and preferably at from 10 000 to 300 000 Dalton.Similarly, the hereinbelow described aqueous solutions of other polymerscontaining amino and/or ammonium groups can be obtained in salt-freeform by ultrafiltration.

Similarly, derivatives of polymers containing vinylamine units can beused as polymers forming basic hydrogels. For instance, polymerscontaining vinylamine units can be subjected to amidation, alkylation,sulfonamide formation, urea formation, thiourea formation, carbamateformation, acylation, carboxymethylation, phosphonomethylation orMichael addition of the amino groups of the polymer to prepare amultiplicity of suitable hydrogel derivatives. Of particular interesthere are uncrosslinked polyvinylguanidines which are accessible byreaction of polymers containing vinylamine units, preferablypolyvinylamines, with cyanamide (R¹R²N—CN, where R¹, R²═H, C1-C4-alkyl,C3-C6-cycloalkyl, phenyl, benzyl, alkyl-substituted phenyl or naphthyl)cf. U.S. Pat. No. 6,087,448 column 3 line 64 to column 5 line 14.

Polymers containing vinylamine units further include hydrolyzed graftpolymers of for example N-vinylformamide on polyalkylene glycols,polyvinyl acetate, polyvinyl alcohol, polyvinylformamides,polysaccharides such as starch, oligosaccharides or monosaccharides. Thegraft polymers are obtainable for example by free-radically polymerizingN-vinylformamide in an aqueous medium in the the presence of at leastone of the grafting bases mentioned, optionally together withcopolymerizable other monomers, and subsequently hydrolyzing theengrafted vinylformamide units in a known manner to obtain vinylamineunits.

Useful water-absorbing basic polymers further include polymers ofdialkylaminoalkyl(meth)acrylamides. Useful monomers for preparing suchpolymers include for example dimethylaminoethylacrylamide,dimethylaminoethylmethacrylamide, dimethylaminopropylacrylamide,dimethylaminopropylmethacrylamide, diethylaminoethylacrylamide,diethylaminoethylmethacrylamide and diethylaminopropylacrylamide. Thesemonomers may be used in the form of the free bases, as salts withinorganic or organic acids or in quaternized form in the polymerization.They may be free-radically polymerized to homopolymers or together withother copolymerizable monomers to copolymers. The polymers contain forexample at least 30 mol % and preferably at least 70 mol % of unitsderived from the basic monomers mentioned. Water-absorbing basicpolymers based on poly(dimethylaminoalkylacrylamide)s are known fromU.S. Pat. No. 5,962,578.

Useful basic polymers further include polyethyleneimines, which arepreparable for example by polymerization of ethyleneimine in aqueoussolution in the presence of acid-detaching compounds, acids or Lewisacids as a catalyst. Polyethyleneimines have for example molar masses ofup to 2 million and preferably from 200 to 1 000 000. Particularpreference is given to using polyethyleneimines having molar masses from500 to 750 000. The polyethyleneimines may optionally be modified, forexample alkoxylated, alkylated or amidated. They may also be subjectedto a Michael addition or a Stecker synthesis. The polyethyleneiminederivatives obtainable thereby are likewise useful as basic polymers forpreparing water-absorbing basic polymers.

Useful basic polymers further include ethyleneimine-graftedpolyamidoamines, which are obtainable for example by condensingdicarboxylic acids with polyamines and subsequent grafting withethyleneimine. Useful polyamidoamines are obtained for example byreacting dicarboxylic acids having 4 to 10 carbon atoms withpolyalkylenepolyamines containing 3 to 10 basic nitrogen atoms in themolecule. Examples of dicarboxylic acids are succinic acid, maleic acid,adipic acid, glutaric acid, suberic acid, sebacic acid and terephthalicacid. Polyamidoamines may also be prepared using mixtures ofdicarboxylic acids and/or mixtures of a plurality ofpolyalkylenepolyamines. Useful polyalkylenepolyamines include forexample diethylenetriamine, triethylenetetramine,tetraethylenepentamine, dipropylenetriamine, tripropylenetetramine,dihexamethylenetriamine, aminopropylethylenediamine andbisaminopropylethylenediamine. To prepare polyamidoamines, thedicarboxylic acids and polyalkylenepolyamines are heated tocomparatively high temperatures, for example to temperatures in therange from 120 to.220° C. and preferably in the range from 130 to 180°C. The water formed in the course of the condensation is removed fromthe system. The condensation may optionally also utilize lactones orlactams of carboxylic acids having 4 to 8 carbon atoms. The amount ofpolyalkylenepolyamine used per mole of a dicarboxylic acid is forexample in the range from 0.8 to 1.4 mol. These polyamidoamines aregrafted with ethyleneimine. The grafting reaction is carried out forexample in the presence of acids or Lewis acids such as sulfuric acid orboron trifluoride etherates at for example from 80 to 100° C. Compoundsof this kind are described in DE-B-24 34 816 for example.

Useful basic polymers further include the optionally crosslinkedpolyamidoamines, which may additionally have been grafted withethyleneimine prior to any crosslinking. The crosslinkedethyleneimine-grafted polyamidoamines are water soluble and have forexample an average molecular weight from 3000 to 2 million Dalton.Customary crosslinkers include for example epichlorohydrin orbischlorohydrin ethers of alkylene glycols and polyalkylene glycols.

Useful basic polymers further include polyallylamines. Polymers of thiskind are obtained by homopolymerization of allylamine, preferably inacid-neutralized form, or by copolymerizing allylamine with othermonoethylenically unsaturated monomers described above as comonomers forN-vinylcarboxamides.

Useful basic polymers further include water-soluble crosslinkedpolyethyleneimines which are obtainable by reaction ofpolyethyleneimines with crosslinkers such as epichlorohydrin orbischlorohydrin ethers of polyalkylene glycols having from 2 to 100ethylene oxide and/or propylene oxide units and which still have freeprimary and/or secondary amino groups. Also suitable are amidicpolyethyleneimines which are obtainable for example by amidation ofpolyethyleneimines with C₁-C₂₂-monocarboxylic acids. Useful cationicpolymers further include alkylated polyethyleneimines and alkoxylatedpolyethyleneimines. The polyethyleneimine is alkoxylated using forexample from 1 to 5 ethylene oxide or propyleneoxide units per NH unitin the polyethyleneimine.

The abovementioned basic polymers have for example K values from 8 to300 and preferably from 15 to 180 (determined after H. Fikentscher in 5%aqueous sodium chloride solution at 25% and a polymer concentration of0.5% by weight). At pH 4.5 their charge density is for example not lessthan 1 and preferably not less than 4 meq/g of polyelectrolyte.

Preferred basic polymers include polymers containing vinylamine units,polyvinylguanidines and polyethyleneimines. Examples thereof are:

vinylamine homopolymers, 10-100% hydrolyzed polyvinylformamides,partially or completely hydrolyzed copolymers of vinylformamide andvinyl acetate, vinyl alcohol, vinylpyrrolidone or acrylamide each havingmolar masses of 3000-2 000 000 and also polyethyleneimines, crosslinkedpolyethyleneimines or amidated polyethyleneimines which each have molarmasses from 500 to 3 000 000. The polymer content of the aqueoussolution is for example, from 1 to 60%, preferably from 2 to 15% andusually from 5 to 10% by weight.

(b) Crosslinkers

To convert the above-described basic polymers into water-absorbing basicpolymers, they are reacted with at least one crosslinker. The basicpolymers are usually soluble or readily dispersible in water.Crosslinking is therefore mainly carried out in an aqueous medium.Preference is given to using aqueous solutions of basic polymers thathave been desalted, for example by ultrafiltration, or whose neutralsalt content is below 1% or below 0.5% by weight. The crosslinkers haveat least two reactive groups capable of reacting with the amino groupsof the basic polymers to form insoluble products which arewater-absorbing polymers. The amount of crosslinker used per 1 part byweight of basic polymer is for example in the range from 0.1 to 50 partsby weight, preferably in the range from 1 to 5 parts by weight andespecially in the range from 1.5 to 3 parts by weight. Usefulcrosslinkers are described in Wo-A-00/63295 page 14 line 43 to page 21line 5.

Useful bi- or polyfunctional crosslinkers include for example

-   -   (1) di- and polyglycidyl compounds    -   (2) di- and polyhalogen compounds    -   (3) compounds having two or more isocyanate groups, which may be        blocked    -   (4) polyaziridines    -   (5) carbonic acid derivatives    -   (6) compounds having two or more activated double bonds capable        of undergoing a Michael addition    -   (7) di- and polycarboxylic acids and acid derivatives thereof    -   (8) monoethylenically unsaturated carboxylic acids, esters,        amides and anhydrides    -   (9) di- and polyaldehydes and di- and polyketones.

Preferred crosslinkers (1) are for example the bischlorohydrin ethers ofpolyalkylene glycols described in U.S. Pat. No. 4 144 123. Phosphoricacid diglycidyl ether and ethylene glycol diglycidyl ether are alsosuitable.

Further crosslinkers are the products of reacting at least trihydricalcohols with epichlorohydrin to form reaction products having at leasttwo chlorohydrin units, polyhydric alcohols used being for exampleglycerol, ethoxylated or propoxylated glycerols, polyglycerols having 2to 15 glycerol units in the molecule and also optionally ethoxylatedand/or propoxylated polyglycerols. Crosslinkers of this type are knownfrom DE-A-2 916 356 for example.

Useful crosslinkers (2) are α,ω- or vicinal dichloroalkanes, for example1,2-dichloroethane, 1,2-dichloropropane, 1,3-dichlorobutane and1,6-dichlorohexane.

Furthermore, EP-A-0 025 515 discloses α,ω-dichloropolyalkylene glycolshaving preferably 1-100, especially 1-100 ethylene oxide, units for useas crosslinkers.

Useful crosslinkers further include crosslinkers (3) which containblocked isocyanate groups, for example trimethylhexamethylenediisocyanate blocked with 2,2,6,6-tetramethylpiperidin-4-one. Suchcrosslinkers are known; cf. for example from DE-A-4 028 285.

Preference is further given to crosslinkers (4) which contain aziridineunits and are based on polyethers or substituted hydrocarbons, forexample 1,6-bis-N-aziridinomethane, cf. U.S. Pat. No. 3,977,923. Thisclass of crosslinkers further includes products formed by reactingdicarboxylic esters with ethyleneimine and containing at least twoaziridino groups, and mixtures thereof.

Useful halogen-free crosslinkers of group (4) include reaction productsprepared by reacting ethyleneimine with dicarboxylic esters completelyesterified with monohydric alcohols of from 1 to 5 carbon atoms.Examples of suitable dicarboxylic esters are dimethyl oxalate, diethyloxalate, dimethyl succinate, diethyl succinate, dimethyl adipate,diethyl adipate and dimethyl glutarate. For instance, reacting diethyloxalate with ethyleneimine gives bis[β-(1-aziridino)ethyl]oxalamide.Dicarboxylic esters are reacted with ethyleneimine in a molar ratio of1: at least 4. The reactive groups of these crosslinkers are theterminal aziridine groups. These crosslinkers may be characterized forexample with the aid of the formula:

where n is from 0 to 22.

Illustrative of crosslinkers (5) are ethylene carbonate, propylenecarbonate, urea, thiourea, guanidine, dicyandiamide or 2-oxazolidinoneand its derivatives. Of this group of monomers, preference is given tousing propylene carbonate, urea and guanidine.

Crosslinkers (6) are reaction products of polyetherdiamines,alkylenediamines, polyalkylenepolyamines, alkylene glycols, polyalkyleneglycols or mixtures thereof with monoethylenically unsaturatedcarboxylic acids, esters, amides or anhydrides of monoethylenicallyunsaturated carboxylic acids, which reaction products contain at leasttwo ethylenically unsaturated double bonds, carboxamide, carboxyl orester groups as functional groups, and also methylenebisacrylamide anddivinyl sulfone.

Crosslinkers (6) are for example reaction products of polyetherdiamineshaving preferably from 2 to 50 alkylene oxide units, alkylenediaminessuch as ethylenediamine, propylenediamine, 1,4-diaminobutane and1,6-diaminohexane, polyalkylenepolyamines having molecular weights <5000for example diethylenetriamine, triethylenetetramine,dipropylenetriamine, tripropylenetetramine, dihexamethylenetriamine andaminopropylethylenediamine, alkylene glycols, polyalkylene glycols ormixtures thereof with

-   -   monoethylenically unsaturated carboxylic acids,    -   esters of monoethylenically unsaturated carboxylic acids,    -   amides of monoethylenically unsaturated carboxylic acids, and    -   anhydrides of monoethylenically unsaturated carboxylic acids.

These reaction products and their preparation are described in EP-A-873371 and are expressly mentioned for use as crosslinkers.

Particularly preferred crosslinkers are the therein mentioned reactionproducts of maleic anhydride with α,ω-polyetherdiamines having a molarmass of from 400 to 5000, the reaction products of polyethyleneimineshaving a molar mass of from 129 to 50 000 with maleic anhydride and alsothe reaction products of ethylenediamine or triethylenetetramine withmaleic anhydride in a molar ratio of 1: at least 2.

Crosslinkers (6) are preferably compounds of the formula

where X, Y, Z=0, NH

-   -   and Y is additionally CH₂        m, n=0−4        p, q=0−45 000,        which are obtainable by reacting polyetherdiamines,        ethylenediamine or polyalkylenepolyamines with maleic anhydride.

Further halogen-free crosslinkers of group (7) are at least dibasicsaturated carboxylic acids such as dicarboxylic acids and also thesalts, diesters and diamides derived therefrom. These compounds may becharacterized for example by means of the formulaX—CO—(CH₂)_(n)—CO—Xwhere X═OH, OR¹, N(R²)₂R¹═C₁-C₂₂-alkyl,R²═H, C₁-C₂₂-alkyl andn=0−22.

As well as dicarboxylic acids of the abovementioned formula it ispossible to use, for example, monoethylenically unsaturated dicarboxylicacids such as maleic acid or itaconic acid. The esters of thecontemplated dicarboxylic acids are preferably derived from alcoholshaving from 1 to 4 carbon atoms. Examples of suitable dicarboxylicesters are dimethyl oxalate, diethyl oxalate, diisopropyl oxalate,dimethyl succinate, diethyl succinate, diisopropyl succinate,di-n-propyl succinate, diisobutyl succinate, dimethyl adipate, diethyladipate and diisopropyl adipate or Michael addition products whichcontain at least two ester groups and are formed from polyetherdiamines,polyalkylenepolyamines or ethylenediamine and esters of acrylic acid ormethacrylic acid with, in each case, monohydric alcohols of from 1 to 4carbon atoms. Examples of suitable esters of ethylenically unsaturateddicarboxylic acids are dimethyl maleate, diethyl maleate, diisopropylmaleate, dimethyl itaconate and diisopropyl itaconate. It is alsopossible to use substituted dicarboxylic acids and their esters such astartaric acid (D,L-form and as racemate) and also tartaric esters suchas dimethyl tartrate and diethyl tartrate.

Examples of suitable dicarboxylic anhydrides are maleic anhydride,itaconic anhydride and succinic anhydride. Useful crosslinkers (7)further include for example dimethyl maleate, diethyl maleate and maleicacid. The crosslinking of amino-containing compounds with theaforementioned crosslinkers takes place with the formation of amidegroups or, in the case of amides such as adipamide, by transamidation.Maleic esters, monoethylenically unsaturated dicarboxylic acids andtheir anhydrides can bring about crosslinking both by formation ofcarboxamide groups and by addition of NH groups of the component to becrosslinked (polyamidoamines, for example) in the manner of a Michaeladdition.

The at least dibasic saturated carboxylic acids of crosslinker class (7)include for example tri- and tetracarboxylic acids such as citric acid,propanetricarboxylic acid, nitrilotriacetic acid,ethylenediaminetetraacetic acid, butanetetracarboxylic acid anddiethylenetriaminepentaacetic acid. Useful crosslinkers of group (7)further include the salts, esters, amides and anhydrides derived fromthe aforementioned carboxylic acids, e.g., dimethyl tartrate, diethyltartrate, dimethyl adipate and diethyl adipate.

Useful crosslinkers of group (7) further include polycarboxylic acidsobtainable by polymerizing monoethylenically unsaturated carboxylicacids, anhydrides, esters or amides. Examples of suitablemonoethylenically unsaturated carboxylic acids are acrylic acid,methacrylic acid, fumaric acid, maleic acid and/or itaconic acid.Examples of useful crosslinkers are accordingly polyacrylic acids,copolymers of acrylic acid and methacrylic acid or copolymers of acrylicacid and maleic acid. Illustrative comonomers are vinyl ether, vinylformate, vinyl acetate and vinyllactam.

Further useful crosslinkers (7) are prepared for example by free-radicalpolymerization of anhydrides such as maleic anhydride in an inertsolvent such as toluene, xylene, ethylbenzene, isopropylbenzene orsolvent mixtures. Besides the homopolymers, copolymers of maleicanhydride are suitable, for example copolymers of acrylic acid andmaleic anhydride or copolymers of maleic anhydride and a C₂- toC₃₀-olefin.

Examples of preferred crosslinkers (7) are copolymers of maleicanhydride and isobutene or copolymers of maleic anhydride anddiisobutene. Copolymers containing anhydride groups may optionally bemodified by reaction with C₁- to C₂₀-alcohols or ammonia or amines andbe used as crosslinkers in that form.

Examples of preferred polymeric crosslinkers (7) are copolymers ofacrylamide and acrylic esters, for example hydroxyethyl acrylate ormethyl acrylate, the molar ratio of acrylamide and acrylic ester varyingin the range from 90:10 to 10:90. Besides these copolymers, terpolymerscan be used, an example of the useful combinations being acrylamide,methacrylamide and acrylate/methacrylate.

The molar mass M_(W) of the homo- and copolymers may be up to 10 000,preferably from 500 to 5000. Polymers of the abovementioned type aredescribed for example in EP-A-0 276 464, U.S. Pat. No. 3,810,834, GB-A-1411 063 and U.S. Pat. No. 4,818,795. The at least dibasic saturatedcarboxylic acids and the polycarboxylic acids may also be used ascrosslinkers in the form of the alkali metal or ammonium salts.Preference is given to using the sodium salts. The polycarboxylic acidsmay be partially neutralized, for example to an extent of from 10 to 50mol %, or else completely neutralized.

Useful halogen-free crosslinkers of group (8) include for examplemonoethylenically unsaturated monocarboxylic acids such as acrylic acid,methacrylic acid and crotonic acid and the amides, esters and anhydridesderived therefrom. The esters may be derived from alcohols of 1 to 22,preferably of from 1 to 18, carbon atoms. The amides are preferablyunsubstituted, but may bear a C₁-C₂₂-alkyl substituent.

Preferred crosslinkers (8) are acrylic acid, methyl acrylate, ethylacrylate, acrylamide and methacrylamide.

Useful halogen-free crosslinkers of group (9) include for exampledialdehydes or their hemiacetals or acetals as precursors, for exampleglyoxal, methylglyoxal, malonaldehyde, succinaldehyde, maleialdehyde,fumaraldehyde, tartaraldehyde, adipaldehyde, 2-hydroxyadipaldehyde,furan-2,5-dipropionaldehyde, 2-formyl-2,3-dihydropyran, glutaraldehyde,pimelaldehyde and also aromatic dialdehydes such as, for example,terephthalaldehyde, o-phthalaldehyde, pyridine-2,6-dialdehyde orphenylglyoxal. But it is also possible to use homo- or copolymers ofacrolein or methacrolein having molar masses of from 114 to about 10000. Useful comonomers include in principle all water-solublecomonomers, for example acrylamide, vinyl acetate and acrylic acid.Aldehyde starches are similarly useful as crosslinkers.

Useful halogen-free crosslinkers of group (9) include for examplediketones or the corresponding hemiketals or ketals as precursors, forexample β-diketones such as acetylacetone or cycloalkane-1,n-diones suchas, for example, cyclopentane-1,3-dione and cyclohexane-1,4-dione. Butit is also possible to use homo- or copolymers of methyl vinyl ketonehaving molar masses of from 140 to about 15 000. Useful comonomersinclude in principle all water-soluble monomers, for example acrylamide,vinyl acetate and acrylic acid.

It will be appreciated that mixtures of two or more crosslinkers mayalso be used.

Preferred crosslinkers are glycidyl ethers of alkylene glycols such asethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol andpolyalkylene glycols having molar masses up to 1500 and also thecompletely acrylated and/or methacrylated addition products of from 1 to25 mol and preferably from 2 to 15 mol of ethylene oxide and 1 mol oftrimethylolpropane or pentaerythritol.

(c) Surfactants

The crosslinkable aqueous mixtures include from 0.1 to 20% by weight ofat least one surfactant as a further component. The surfactants are ofdecisive importance for forming and stabilizing the foam. It is possibleto use anionic, cationic or nonionic surfactants or surfactant mixtureswhich are compatible with each other. It is possible to use lowmolecular weight or else polymeric surfactants, and combinations ofdifferent or else similar types of surfactants have been determined tobe advantageous. Examples of nonionic surfactants are addition productsof alkylene oxides, especially ethylene oxide, propylene oxide and/orbutylene oxide, with alcohols, amines, phenols, naphthols or carboxylicacids. The surfactants used are advantageously addition products ofethylene oxide and/or propylene oxide with alcohols containing at least10 carbon atoms, the addition products containing from 3 to 200 mol ofethylene oxide and/or propylene oxide per mole of alcohol. The alkyleneoxide units are present in the addition products in the form of blocksor in random distribution. Examples of nonionic surfactants are theaddition products of 7 mol of ethylene oxide with 1 mol of tallow fatalcohol, reaction products of 9 mol of ethylene oxide with 1 mol oftallow fat alcohol and addition products of 80 mol of ethylene oxidewith 1 mol of tallow fat alcohol. Further commercially availablenonionic surfactants comprise reaction products of oxo process alcoholsor Ziegler alcohols with from 5 to 12 mol of ethylene oxide per mole ofalcohol, especially with 7 mol of ethylene oxide. Further commerciallyavailable nonionic surfactants are obtained by ethoxylation of castoroil. The amount of ethylene oxide added per mole of castor oil is forexample in the range from 12 to 80 mol. Further commercially availableproducts are for example the reaction products of 18 mol of ethyleneoxide with 1 mol of tallow fat alcohol, the addition products of 10 molof ethylene oxide with 1 mol of a C₁₃/Cl₅ oxo process alcohol or thereaction products of from 7 to 8 mol of ethylene oxide with 1 mol of aC₁₃/Cl₅ oxo process alcohol. Useful nonionic surfactants further includephenol alkoxylates such as for example p-tert-butylphenol which has beenreacted with 9 mol of ethylene oxide or methyl ethers of reactionproducts of 1 mol of a C₁₂-C₁₈ alcohol and 7.5 mol of ethylene oxide.

The nonionic surfactants described above, for example by esterificationwith sulfuric acid, can be converted into the corresponding acidsulfuric esters. The acid sulfuric esters are used in the form of theiralkali metal or ammonium salts as anionic surfactants. Useful anionicsurfactants include for example alkali metal or ammonium salts of acidsulfuric esters of addition products of ethylene oxide and/or propyleneoxide with fatty alcohols, alkali metal or ammonium salts ofalkylbenzenesulfonic acid or of alkylphenol ether sulfates. Products ofthe kind mentioned are commercially available. For example, the sodiumsalt of an acid sulfuric ester of a C₁₃/C₁₅ oxo process alcohol reactedwith 106 mol of ethylene oxide, the triethanolamine salt ofdodecylbenzenesulfonic acid, the sodium salt of alkylphenol ethersulfates and the sodium salt of the acid sulfuric ester or the reactionproduct of 106 mol of ethylene oxide with 1 mol of tallow fat alcoholare commercially available anionic surfactants. Useful anionicsurfactants further include acid sulfuric esters of C₁₃/C₁₅ oxo processalcohols, paraffinsulfonic acids such as C₁₅-alkylsulfonate,alkyl-substituted benzenesulfonic acids and alkyl-substitutednaphthalenesulfonic acids such as dodecylbenzenesulfonic acid anddi-n-butylnaphthalenesulfonic acid and also fatty alcohol phosphatessuch as C₁₅/C₁₈ fatty alcohol phosphate. The polymerizable aqueousmixture can include combinations of a nonionic surfactant and an anionicsurfactant or combinations of nonionic surfactants or combinations ofanionic surfactants. Even cationic surfactants are suitable. Examplesthereof are the dimethyl sulfate quaternized reaction products of 6.5mol of ethylene oxide with 1 mol of oleylamine,distearyldimethylammonium chloride, lauryltrimethylammonium chloride,cetylpyridinium bromide and dimethyl sulfate quaternized triethanolaminestearate, which is preferably used as a cationic surfactant.

The surfactant content of the aqueous mixture is preferably in the rangefrom 0.5 to 10% by weight. In most cases, the aqueous mixtures have asurfactant content from 1.5 to 8% by weight.

(d) Solubilizers

The crosslinkable aqueous mixtures may optionally include at least onesolubilizer as a further component. Solubilizers are water-miscibleorganic solvents, for example dimethyl sulfoxide, dimethylformamide,N-methylpyrrolidone, monohydric alcohols, glycols, polyethylene glycolsor monoethers derived therefrom, subject to the proviso that themonoethers do not contain any double bonds in the molecule. Usefulethers include methylglycol, butylglycol, butyldiglycol, methyldiglycol,butyltriglycol, 3-ethoxy-1-propanol and glycerol monomethyl ether.

The aqueous mixtures include from 0 to 50% by weight of at least onesolubilizer. When solubilizers are used, they are preferably included inthe aqueous mixture in an amount from 1 to 25% by weight.

(e) Thickeners, Foam Stabilizers, Fillers, Fibers, Cell Nucleators

The crosslinkable aqueous mixture may optionally include thickeners,foam stabilizers, fillers, fibers and/or cell nucleators. Thickeners areused for example to optimize foam structure and to improve foamstability. As a result, the foam will shrink only minimally during thepolymerization. Useful thickeners include all natural and syntheticpolymers known for this purpose that substantially increase theviscosity of an aqueous system and do not react with the amino groups ofthe basic polymers. The synthetic and natural polymers in question canbe swellable or soluble in water. An exhaustive overview of thickenersmay be found for example in the publications by R. Y. Lochhead and W. R.Fron, Cosmetics & Toiletries, 108, 95-135 (May 1993) and M. T. Clarke,“Rheological Additives” in D. Laba (ed.) “Rheological Properties ofCosmetics and Toiletries”, Cosmetic Science and Technology Series, Vol.13, Marcel Dekker Inc., New York 1993.

Water-swellable or water-soluble synthetic polymers useful as thickenersinclude for example high molecular weight polyethylene glycols orcopolymers of ethylene glycol and propylene glycol and also highmolecular weight polysaccharides such as starch, guar flour, locust beanflour or derivatives of natural substances such ascarboxymethylcellulose, hydroxyethylcellulose, hydroxymethylcellulose,hydroxypropylcellulose and mixed cellulose ethers. A further group ofthickeners are water-insoluble products, such as finely divided silica,zeolites, bentonite, cellulose powders and other finely divided powdersof crosslinked polymers. The aqueous mixtures may include the thickenersin amounts up to 30% by weight. When such thickeners are used at all,they are included in the aqueous mixture in amounts of 0.1%, preferably0.5% up to 20% by weight.

To optimize foam structure, the aqueous reaction mixture may be admixed,if applicable, with hydrocarbons having at least 5 carbon atoms in themolecule. Useful hydrocarbons include for example pentane, cyclopentane,hexane, cyclohexane, heptane, octane, isooctane, decane and dodecane.The contemplated aliphatic hydrocarbons can be straight-chain, branchedor cyclic and have a boiling temperature which is above the temperatureof the aqueous mixture during foaming. The aliphatic hydrocarbons extendthe pot life of the foamed aqueous reaction mixture which has not yetpolymerized. This facilitates the handling of the foams which have notyet polymerized and increases process consistency. The hydrocarbons actfor example as cell nucleators and also stabilize the foam which hasalready formed. In addition, they can effect a further foaming of themixture in the course of the polymerization of the monomer foam. Theycan then also have the function of a blowing agent. Instead ofhydrocarbons or a mixture therewith, it is also possible to useoptionally chorinated or fluorinated hydrocarbons as a cell nucleatorand/or foam stabilizer, for example dichloromethane, trichloromethane,1,2-dichloroethane, trichlorofluoromethane or1,1,2-trichlorotrifluoroethane. When hydrocarbons are used, they areused for example in amounts from 0.1 to 20% by weight and preferablyfrom 0.1 to 10% by weight, based on the polymerizable aqueous mixture.

To modify the properties of the foams, the crosslinkable aqueous mixturemay have added to it one or more fillers, for example chalk, talc, clay,titanium dioxide, magnesium oxide, aluminum oxide, precipitated silicasin hydrophilic or hydrophobic forms, dolomite and/or calcium sulfate.The particle size of the fillers is for example in the range from 10 to1000 μm and preferably in the range from 50 to 850 μm. Fillers can beincluded in the crosslinkable aqueous mixture in amounts up to 30% byweight.

The properties of the foams can optionally also be modified by means offibers. The fibers in question can be natural or synthetic fibers orfiber blends, for example fibers composed of cellulose, wool,polyethylene, polypropylene, polyesters or polyamides. When fibers areused, they may be present in the aqueous mixture in an amount of forexample up to 200% by weight and preferably up to 25% by weight. Fillersand fibers can optionally also be added to the ready-foamed mixture. Theuse of fibers leads to an enhancement of the strength properties, suchas wet strength, of the ready-produced foam.

(f) Water-Absorbing Acidic Polymers

Useful water-absorbing acidic polymers, hereinafter also referred to asacidic superabsorbents, include all hydrogels described for example inWO-A-00/63295 page 2 line 27 to page 9 line 16. The materials inquestion are essentially lightly crosslinked polymers of acidic monomersthat possess a high water uptake ability when in at least partiallyneutralized form. Examples of such crosslinked polymers, which are eachlightly crosslinked, are crosslinked polyacrylic acids, crosslinkedhydrolyzed graft polymers of acrylonitrile on starch, crosslinked graftpolymers of acrylic acid on starch, hydrolyzed crosslinked copolymers ofvinyl acetate and acrylic esters, crosslinked polyacrylamides,hydrolyzed crosslinked polyacrylamides, crosslinked copolymers ofethylene and maleic anhydride, crosslinked copolymers of isobutylene andmaleic anhydride, crosslinked polyvinylsulfonic acids, crosslinkedpolyvinylphosphonic acids and crosslinked sulfonated polystyrene. Theacidic superabsorbents mentioned can be added to the crosslinkableaqueous mixture either alone or in mixture with each other. The acidicsuperabsorbents used are preferably particulate polymers of neutralizedpolyacrylic acids which are lightly crosslinked. The acid groups of theacidic superabsorbents are preferably neutralized with aqueous sodiumhydroxide solution, with sodium bicarbonate or with sodium carbonate.The neutralization can also be effected, however, with aqueous potassiumhydroxide solution, ammonia, amines or alkanolamines such asethanolamine, diethanolamine or triethanolamine.

The water-absorbing acidic polymers are added in particulate form to thecrosslinkable mixture or preferably to the already foamed crosslinkablemixture. The particles can be used in solid form or in foamed form. Theweight average particle diameter is for example in the range from 10 to2000 μm, preferably in the range from 100 to 850 μm and usually in therange from 150 to 450 μm. Superabsorbents having the appropriateparticle sizes can be prepared for example by comminution, for exampleby grinding, of coarsely granular, solid superabsorbents or of foamedsuperabsorbents. The density of the foamed acidic superabsorbents is forexample in the range from 0.01 to 0.9 g/cm³ and preferably in the rangefrom 0.05 to 0.7 g/cm³. The surface of the particulate superabsorbentscan have been postcrosslinked, if desired. It is preferable to useacidic superabsorbents whose surface has not been postcrosslinked.

Acidic superabsorbents are known from the above-cited references, cf. inparticular WO-A-00/63295 page 6 line 36 to page 7 line 44. Surfacepostcrosslinking is effected, for example, by reacting particles oflightly crosslinked polyacrylic acids with compounds having at least twocarboxyl-reactive groups. The compounds in question are typicalcrosslinkers which were indicated above under (b). Compounds which areof particular interest for use as crosslinkers include for example,polyhydric alcohols such as propylene glycol, 1,4-butanediol or1,6-hexanediol and glycidyl ethers of ethylene glycol and polyethyleneglycols having molar masses from 200 to 1500 and preferably from 300 to400 and completely acrylated or methacrylated reaction products oftrimethylolpropane, of reaction products formed from trimethylolpropaneand ethyleneoxide in a molar ratio from 1:1 to 1:25 and preferably from1:3 to 1:15 and also of reaction products of pentaerythritol withethylene oxide in a molar ratio of 1:30 and preferably a molar ratiofrom 1:4 to 1:20. The postcrosslinking of the surface of the anionicsuperabsorbent particles is carried out for example at up to 220° C.,for example preferably in the range from 120 to 190° C.

The water-absorbing acidic polymers used are superabsorbents in the formof particles having the above-indicated particle sizes. Whenwater-absorbing acidic polymers are incorporated into the crosslinkableaqueous mixture, the polymer mixture will include for example from 10 to90% and preferably from 30 to 70% by weight of a water-absorbing acidicpolymer. The mixture of foamed basic hydrogel and the optionally foamedacidic hydrogel will usually include from 40 to 60% by weight of theacidic superabsorbent.

To prepare foams which have a high absorptive ability even for salineaqueous solutions, the basic and acidic superabsorbents are preferablyused in unneutralized form. The degree of neutralization of the acidicwater-absorbing polymers is for example from 0 to 100, preferably from 0to 75 and usually from 0 to 50 mol %. The water-absorbing basic polymershave a higher uptake capacity for saline aqueous solutions andespecially acidic aqueous solutions when in the form of the free basesthan in acid-neutralized form. When basic polymers are used as solewater-absorbing polymers, the degree of neutralization is for examplefrom 0 to 100 and preferably from 0 to 60 mol %.

Preparing the Foam

The above-described crosslinkable aqueous mixture, which includes (a) abasic polymer, (b) a crosslinker and (c) a surfactant as mandatorycomponents and also at least one of the optional components (d), (e)and/or (f), is initially foamed. For example, an inert gas can bedissolved in the crosslinkable aqueous mixture at a pressure of forexample 2-400 bar and the mixture subsequently decompressed toatmospheric. Decompression from a nozzle produces a flowable foam. Thecrosslinkable aqueous mixture can also be foamed by another method,namely by dispersing fine bubbles of an inert gas in the crosslinkableaqueous mixture. The foaming of the crosslinkable aqueous mixture on alaboratory scale can be effected for example by foaming the aqueousmixture in a kitchen processor equipped with a whisk. Foaming ispreferably carried out in an inert gas atmosphere, for example innitrogen or noble gases under atmospheric or superatmospheric pressure,for example up to 25 bar, and subsequent decompression. The consistencyof the foams, the size of the gas bubbles and the distribution of thegas bubbles in the foam can be varied in wide limits, for examplethrough the choice of surfactants, solubilizers, foam stabilizers, cellnucleators, thickeners and fillers. As a result, the density, theopen-cell content of the foam and the wall thickness of the foam arereadily adjustable to specific values. The aqueous mixture is preferablyfoamed at temperatures which are below the boiling point of theconstituents of the aqueous mixture, for example in the range from roomtemperature to 100° C. and preferably in the range from 20 to 50° C.However, the aqueous mixture can also be foamed at temperatures abovethe boiling point of the component having the lowest boiling point byfoaming the mixture in a pressure tightly sealed container. The foamsobtained are crosslinkable mixtures which are flowable and stable for aprolonged period. The density of the foamed crosslinkable mixture is forexample in the range from 0.01 to 0.9 g/cm³ at 20° C.

Crosslinking the Foamed Mixture

The second step of the process comprises crosslinking the basic polymerto form a water-absorbing basic polymer. The originally water-solublepolymer is rendered water-insoluble by crosslinking. A hydrogel of abasic polymer is obtained. The crosslinkable foams are for exampletransferred into suitable molds and heated therein, so that thecrosslinkers react with the basic polymer. The foamed material can beapplied for example in the desired thickness to a temporary carriermaterial which advantageously has been provided with an antistickcoating. The foam can be knife coated onto a support for example.Another possibility is to fill the aqueous foam mix into molds whichhave likewise been antistick coated.

Since the foamed aqueous mixture has a long pot life, this mixture isalso suitable for producing composite materials. For example, it can beapplied to a permanent carrier material, for example polymeric films(films of polyethylene, polypropylene or polyamide for example) or metalsuch as aluminum foils. The foamed aqueous mixture can also be appliedto nonwovens, fluff, tissues, wovens, natural or synthetic fibers orother foams. To prepare composite materials, it may be preferable toapply the foam in the shape of defined structures or in different layerthickness to a carrier material. However, it is also possible to applythe foam to fluff layers or nonwovens and to impregnate these materialsin such a way that the fluff becomes an integral part of the foam aftercrosslinking. The foamed aqueous mixture obtainable in the first processstep can also be molded into large blocks before crosslinking. Aftercrosslinking, the blocks can be cut or sawed into smaller articles. Itis also possible to prepare sandwich like structures by applying afoamed aqueous mixture to a support, covering the foam layer with afilm, foil, nonwoven, tissue, woven, fibers or other foam andcrosslinking the sandwich like structure by heating. However, it is alsopossible, before or after crosslinking, to apply at least one furtherlayer composed of a foamed crosslinkable layer and if desired cover itwith a further film, foil, nonwoven, tissue, woven, fibers or othermaterials. The composite is then subjected to crosslinking in the secondprocess step. However, it is also possible to prepare sandwich likestructures having further foam layers of the same density or differentdensities.

Inventive foam layers having a layer thickness of up to about 1millimeter are prepared for example by heating one side or in particularby irradiating one side of the foamed crosslinkable aqueous mixture.When thicker layers of a foam are to be produced, for example foamshaving thicknesses of two or more centimeters, it is particularlyadvantageous to heat the crosslinkable foamed material by means ofmicrowaves, since relatively uniform heating can be obtained in thisway. In this case, the crosslinking is effected for example at from 20to 180° C., preferably in the range from 20 to 100° C. and especially inthe range from 65 to 80° C. When thicker foam layers are to becrosslinked, the foamed mixture is heat treated on both surfaces, forexample using contact heating or by irradiation. The density of thebasic hydrogel foams is essentially equal to the density of thecrosslinkable aqueous mixture. Foams of water-absorbing basic polymersare accordingly obtained in a density of for example from 0.01 to 0.9g/cm³ and preferably from 0.1 to 0.7 g/cm³. The basic polymer foams areopen celled. The open-cell content is for example at least 80% andpreferably above 90%. Particular preference is given to foams having anopen-cell content of 100%. The open-cell content of the foam isdetermined using scanning electron microscopy for example.

Foams having a particularly high water uptake capacity and an improveduptake ability for electrolyte-containing aqueous solutions areobtainable by crosslinking foamed aqueous mixtures which, based on thepolymer mixture, include from 10 to 90% by weight of a finely dividedwater-absorbing acidic polymer. The acidic hydrogel can be present inthe foams of the invention as a solid particulate polymer or as a foamedparticulate polymer having particle sizes of for example 10-2000 μm.

After the crosslinking of the foamed mixture or during the crosslinking,the hydrogel foam is dried. This removes water and other volatileconstituents from the crosslinked hydrogel foam. Preferably, thehydrogel foam is dried after it has been crosslinked. Examples ofsuitable drying processes are thermal convection drying, for exampletray, chamber, duct, flat sheet, disk, rotary drum, free fall tower,foraminous belt, flow, fluidized bed, moving bed, paddle and ball beddrying, thermal contact drying such as hotplate, drum, belt, foraminouscylinder, screw, tumble and contact disk drying, radiative drying suchas infrared drying, dielectric drying such as microwave drying andfreeze drying. To avoid unwelcome decomposition and crosslinkingreactions, it may be advantageous to dry under reduced pressure, under aprotective gas atmosphere and/or under benign thermal conditions wherethe product temperature does not exceed 120° C., preferably 100° C.Particularly suitable drying processes are (vacuum) belt drying andpaddle drying.

After drying, the hydrogel foam will usually no longer contain anywater. However, the water content of the foamed material can be adjustedto any desired value by moistening the foam with liquid water or watervapor. The water content of the gel foam is usually in the range from 1to 60% by weight and preferably in the range from 2 to 10% by weight.The water content can be used to adjust the flexibility of the hydrogelfoam. Completely dried hydrogel foams are rigid and brittle, whereasfoamed materials having a water content of for example 5-20% by weightare flexible. The foamed hydrogels can either be used directly in theform of sheets or granules or cut into individual plates or sheets fromthicker blocks.

However, the hydrogel foams described above can additionally be modifiedto the effect that the surface of the foamed materials ispostcrosslinked. This is a way of improving the gel stability of thearticles formed from the foamed hydrogels. To perform surfacepostcrosslinking, the surface of the articles formed from the foamedhydrogels is treated with at least one crosslinking agent and the thustreated articles are heated to a temperature at which the crosslinkerswill react with the hydrogels. Suitable crosslinkers are described aboveunder (b). These compounds can likewise be used for postcrosslinking thesurface of the hydrogel foams. Crosslinkers which are preferably usedare the hereinabove mentioned glycidyl ethers and esters of acrylic acidand/or methacrylic acid with the reaction products of 1 mol oftrimethylolpropane and from 6 to 15 mol of ethylene oxide.

The crosslinkers for the surface postcrosslinking are preferably appliedto the foam surface in the form of an aqueous solution. The aqueoussolution can include water-miscible organic solvents, for examplealcohols such as methanol, ethanol and/or i-propanol or ketones such asacetone. The amount of crosslinker applied to the surface of thehydrogel foams is for example in the range from 0.1 to 5% by weight andpreferably in the range from 1 to 2% by weight. The surfacepostcrosslinking of the hydrogel foams is effected by heating thehydrogel foams which have been treated with at least one crosslinker toa temperature which is for example in the range from 60 to 120° C. andpreferably in the range from 70 to 100° C. After surface crosslinking,the water content of the foamed surface-postcrosslinked hydrogel canlikewise be adjusted to values from 1 to 60% by weight.

The optionally surface-postcrosslinked hydrogel foams of the inventioncan be used for all the purposes for which for example thewater-absorbing hydrogel foams which are known from EP-B-0 858 478 andwhich are based on acid group containing polymers such as crosslinkedpolyacrylates are used. The hydrogel foams of the invention are usefulfor example in hygiene articles to absorb body fluids, in dressingmaterial to cover wounds, as a sealing material, as a packagingmaterial, as a soil improver, as a soil substitute, to dewater sludges,to absorb aqueous acidic wastes, to thicken waterborne paints orcoatings in the course of disposing of residual quantities thereof, todewater water-containing oils or hydrocarbons or as a material forfilters in ventilation systems.

Of particular importance is the use of the hydrogel foams of theinvention in hygiene articles, such as baby diapers, sanitary napkinsand incontinence articles, and in dressing material. In hygiene articlesfor example they perform more than one function, namely acquire,distribute and/or store body fluids. The surface of the hydrogel foamscan optionally be modified by treatment with surfactants or polymerscontaining uncrosslinked vinylamine units. This provides an improvementin the acquisition of fluids.

Layers of the hydrogel foams according to the invention can be forexample disposed in a thickness from 1 to 5 mm in one of theabovementioned hygiene articles as an absorbent core between aliquid-pervious topsheet and a liquid-impervious layer composed of afilm of for example polyethylene or polypropylene. The liquid-perviouslayer of the hygiene article is in use in direct contact with the skinof the user. This material is customarily composed of a nonwoven ofnatural fibers such as cellulose fibers or fluff. If desired, a tissuelayer will be disposed above and/or below the absorbent core. Betweenthe bottom layer of the hygiene article and the absorbent core, theremay optionally be a storage layer composed of a conventional particulateanionic superabsorbent. When the foamed basic hydrogels are used as anabsorbent core in diapers, the open-cell structure of the foamed basichydrogel will ensure that the body fluid, which is normally applied inindividual amounts all at once, is speedily removed. This gives the usera pleasant sense of the surface dryness of the diaper.

Methods of Determination

Density

Any suitable gravimetric method can be used for determining the densityof the multicomponent foam system. What is determined is the mass ofsolid multicomponent foam system per unit volume of foam structure. Amethod for density determination of the multicomponent foam system isdescribed in ASTM Method No. D 3574-86, Test A. This method wasoriginally developed for the density determination of urethane foams,but can also be used for this purpose. By this method, the dry mass andvolume of a preconditioned sample is determined at 22±2° C. Volumedetermination of larger sample dimensions are carried out underatmospheric pressure.

Free Swell Capacity (FSC)

This method is used to determine the free swellability of themulticomponent foam system in a teabag. To determine FSC, 0.2000±0.0050g of dried foam is introduced into a teabag 60×85 mm in size, which issubsequently sealed shut. The teabag is placed in an excess of testsolution (at least 0.83 1 of sodium chloride solution/l g of polymerpowder) for 30 minutes. The teabag is subsequently allowed to drip for10 minutes by being hung up by one corner. The amount of liquid isdetermined by weighing back the teabag.

The test solution used was 0.9% by weight NaCl solution.

Acquisition Time:

The multicomponent foam system is cut into layers 1.5 mm, 2 mm or 4 mmin thickness. A commercially available diaper is carefully cut open, thehigh loft used as acquisition medium removed and instead themulticomponent foam system 7×7 cm in size inserted. The diaper isresealed. The application of 0.9% sodium chloride solution is effectedthrough a plastic plate having a ring in the middle (inner diameter ofthe ring 6.0 cm, height 4.0 cm). The plate is loaded with a weight sothat the total load on the diaper is 13.6 g/cm². The plastic plate isplaced on the diaper in such a way that the center of the diaper is alsothe center of the application ring. Three 10 ml lots of 0.9% NaClsolution are applied. The 0.9% NaCl solution is measured out in agraduated cylinder and applied to the diaper in a continuous streamthrough the ring in the plate. At the same time the time is taken forthe solution to penetrate completely into the diaper. The time measuredis noted as Acquisition Time 1. Thereafter, the diaper is loaded with aplate for 10 minutes, the load being maintained at 13.6 g/cm².Thereafter, the plate is removed, 10 g±0.5 g of Schleicher & Schuell S&S2282, 10×10 cm filter paper are placed on the midpoint and loaded with aweight of 1200 g for 15 s. After this period, the weight is removed andthe filter paper is weighed back. The weight difference is noted asRewet 1. Thereafter, the plastic plate with application ring is againplaced on the diaper and the second application of liquid takes place.The measured time is noted as Acquisition Time 2. The procedure isrepeated as described. This gives Rewets 2 and 3 and also AcquisitionTime 3.

Centrifuge Retention Capacity (CRC)

This method is used to determine the free swellability of themulticomponent foam system in a teabag. To determine CRC, 0.2000±0.0050g of dried multicomponent foam is introduced into a teabag 60×85 mm insize, which is subsequently sealed shut. The teabag is placed in anexcess of 0.9% by weight sodium chloride solution (at least 0.83 1 ofsodium chloride solution/i g of polymer powder) for 30 minutes. Theteabag is then centrifuged at 250 G for 3 minutes. The amount of liquidis determined by weighing back the centrifuged teabag.

The test solution used was 0.9% by weight NaCl solution.

Free Swell Rate (FSR)

To determine the free swell rate, 0.50 g (W_(H)) of the multicomponentfoam system is placed on the base of a plastic dish having a roundbottom of about 6 cm. The plastic dish is about 2.5 cm deep and has asquare opening of about 7.5 cm×7.5 cm. A funnel is then used to add 10 g(W_(U)) of a 0.9% NaCl solution into the center of the plastic dish. Assoon as the liquid has contact with the multicomponent foam system, awatch is started and not stopped until the multicomponent foam systemhas completely taken up the entire liquid, ie until pooled liquid isabsent. This time is noted as t_(A). The free swell rate then computesfromFSR=W _(U)/(W _(H) ×t _(A)).Absorbency Against Pressure (AAP) (0.3 psi)

The measuring cell for determining AAP 0.3 psi is a Plexiglas cylinder60 mm in internal diameter and 50 mm in height. Adhesively attached toits underside is a stainless steel sieve bottom having a mesh size of 36μm. The measuring cell further includes a plastic plate 59 mm indiameter and a weight which can be placed into the measuring celltogether with the plastic plate. The weight of the plastic plate and theweight together correspond to a weight loading of 0.3 psi. AAP 0.3 psiis determined by determining the weight of the empty Plexiglas cylinderand of the plastic plate and recording it as W₀. A piece of themulticomponent foam system 20 mm in diameter is then placed into thePlexiglas cylinder and weighed in. The plastic plate is then carefullyplaced in the Plexiglas cylinder and the entire unit is weighed and theweight is recorded as W_(a). The weight is then placed on the plasticplate in the Plexiglas cylinder. A ceramic filter plate 120 mm indiameter and 0 in porosity is placed in the middle of a Petri dish 200mm in diameter and 30 mm in height and sufficient 0.9% by weight sodiumchloride solution is introduced for the surface of the liquid to belevel without the surface of the filter plate being wetted. A roundfilter paper 90 mm in diameter and <20 μm in pore size (S&S 589Schwarzband from Schleicher & Schüll) is subsequently placed on theceramic plate. The Plexiglas cylinder containing the multicomponent foamsystem is then placed with the plastic plate and weight on top of thefilter paper and left there for 60 minutes. At the end of this period,the complete unit is removed from the Petri dish and filter paper andsubsequently the weight is removed from the Plexiglas cylinder. ThePlexiglas cylinder containing swollen multicomponent foam system isweighed back together with the plastic plate and the weight is recordedas W_(b).

Absorbency against pressure (AAP) is calculated by the followingequation:AAP 0.3 psi [g/g]=[W _(b) −W _(a) ]/[W _(a) −W ₀]

K value

The K value was determined after H. Fikentscher, Cellulose-Chemie,Volume 13, 52-63 and 71-74 (1932) in 5% by weight aqueous solution at pH7, 25° C. and a polymer concentration of 0.5% by weight.

Stiffness

Tensile strength

Toughness

Nominal strain at max load

Nominal strain at break

were each determined in accordance with DIN/EN ISO 527-1, the samplespecimen used satisfying the condition of DIN 53448/A, the clampedlength was 30 mm and the test speed was 6 mm/min.

Unless suggested otherwise by the context, the percentages in theexamples are by weight.

EXAMPLE 1

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 15 g of a 15% aqueous solution of acommercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker. Thecrosslinkable mixture was foamed in the shearing zone of an Ultraturraxstirrer for 1 minute and then poured onto a Teflon support rimmed withaluminum. The mold containing the foamed crosslinkable mixture wasstored at 70° C. in a drying cabinet overnight. During this time, thepolyvinylamine became crosslinked and the foam completely dried. Thehydrogel foam obtained was subsequently adjusted to a water content of5%. It had the properties indicated in Table 1. TABLE 1 AbsorbencyAgainst Pressure (AAP) 13.8 g/g Free Swell Capacity (FSC) 24.4 g/gCentrifuge Retention Capacity (CRC) 12.7 g/g Free Swell Rate (FSR) <0.05g/g · sec.

EXAMPLE 2

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 15 g of a 15% aqueous solution of acommercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker. Inaddition, mixtures were prepared that included the amounts of pentaneindicated in Table 2.

The crosslinkable mixtures were each foamed in the shearing zone of anUltraturrax stirrer for 1 minute and then poured onto a Teflon supportrimmed with aluminum. The foam layer in the mold was 6 mm deep. The moldcontaining the foamed crosslinkable mixture was stored at 70° C. in adrying cabinet overnight. During this time, the polyvinylamine becamecrosslinked and the foam completely dried. The hydrogel foam obtained ineach case was subsequently adjusted to a water content of 5%. They hadthe properties indicated in Tables 2 and 3. TABLE 2 AAP @ Wt % ofDensity 0.3 psi Teabag CRC FSR pentane g/cm³ g/g g/g g/g g/g/sec 0%pentane 0.34 13.8 24.4 12.7 <0.05 2.5% pentane 0.16 7.3 12.8 7.8 0.06 5%pentane 0.18 7.8 44.8 14.9 0.3 10% pentane 0.18 16.5 33.0 11.5 2.8

TABLE 3 0% Pentane 10% Pentane Properties at 5% water content StiffnesskPa 220 ± 28 420 ± 70 Tensile strength kPa 86 ± 2.7 130 ± 13 ToughnessJ/m² 3050 ± 350 4000 ± 780 Nominal strain at max 150 ± 7.5 140 ± 19 load% Nominal strain at 160 ± 7.8 145 ± 19 break % Properties in swollenstate Stiffness kPa 56 ± 9.1 140 ± 11 Tensile strength kPa 25 ± 1.5 25 ±9.2 Toughness J/m² 353 ± 42 200 ± 69 Nominal strain at max 51 ± 5.4 14 ±4 load % Nominal strain at 51 ± 5.6 14 ± 4 break %

EXAMPLE 3

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 15 g of a 15% aqueous solution of acommercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker. Inaddition, mixtures were prepared that included the amounts of cellulosefibers (Technocel fibers 1000, length of fibers 1000 μm) indicated inTable 4. The aqueous crosslinkable mixtures were each then homogenizedat a stirrer speed of 200 rpm for 5 minutes and thereafter foamed at 750rpm for 5 minutes by introducing nitrogen at a rate of 100 l/h.

The foamy mixtures thus prepared were then each poured onto a Teflonsupport rimmed with aluminum. The foam layer in the molds was 6 mm deep.The molds containing the foamed crosslinkable mixtures were stored at70° C. in a drying cabinet overnight. During this time, thepolyvinylamine became crosslinked and the foam completely dried. Thefiber-containing hydrogel foam obtained was subsequently adjusted to awater content of 5%. They had the properties indicated in Tables 4 and5. TABLE 4 Cellulose AAP@ fibers Density 0.3 psi FSC CRC FSR [%] g/cm³g/g [g/g] [g/g] g/g/cm³  0% 0.34 13.8 24.4 12.7 <0.05 25% 0.35 10.7 23.65.6 4 50% 0.41 10.6 23.8 4.5 <0.05 100%  0.42 10.6 21.1 3.6 <0.06 200% 0.43 4.4 8.2 2.8 <0.06

TABLE 5 Cellulose fiber content of crosslinkable mixture 0% 25% 50% 200%Properties of hydrogel foams at 5% water content Stiffness kPa 220 ± 281100 ± 180 1800 ± 270 6200 ± 500 Tensile 86 ± 2.7 240 ± 15 330 ± 14 400± 110 strength kPa Toughness J/m² 3050 ± 350 2100 ± 160 1600 ± 140 1100± 260 Nominal strain 150 ± 7.5 51.3 ± 1 27 ± 2.2 9 ± 1.6 at max load %Nominal strain 160 ± 7.8 58.3 ± 0.89 32 ± 2.7 20 ± 5.1 at break %Properties of hydrogel foams in swollen state Stiffness kPa 56 ± 9.1 140± 16 310 ± 22 850 ± 94 Tensile 25 ± 1.5 34.5 ± 0.22 50 ± 2.5 82 ± 8.6strength kPa Toughness J/m² 353 ± 42 221 ± 11 180 ± 19 180 ± 24 Nominalstrain 51 ± 5.4 34.9 ± 1.1 18.7 ± 0.87 11.8 ± 0.83 at max load % Nominalstrain 51 ± 5.6 36 ± 1.4 20 ± 1.5 20 ± 3 at break %

EXAMPLE 4

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 15 g of a 15% aqueous solution of acommercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker. Theconstituents of the mixture were each stirred, initially at 200 rpm for5 minutes and then at 750 rpm again for 5 minutes, while a 100 l/hnitrogen stream was passed through, to form a crosslinkable foam.

The foam thus prepared was then poured onto a Teflon support rimmed withaluminum. The foam layer in the molds was 6 mm deep. The mold containingthe foamed crosslinkable mixture was stored at 70° C. in a dryingcabinet overnight. During this time, the polyvinylamine becamecrosslinked and the foam completely dried. Samples of the foam thusprepared were adjusted to a water content of 5%. Thereafter, each of theacids indicated in Table 6 were sprayed onto the foam samples, so thatthe foam surface had a pH in the acidic range. Thereafter, theflexibility of the foams was evaluated. The results are indicated inTable 6. TABLE 6 Degree of Foam treated neutralization FSC CRCFlexibility with [%] pH [g/g] [g/g] at 20% RH — 0 10.0 25 13 FlexibleHCl 75 4.3 48 27 Rigid Methanesulfonic 75 3.6 32 17 Flexible acid Lacticacid 75 5.6 31 13 Flexible Citric acid 75 3.3 3 1 Rigid Hydroxysuccinic75 3.2 4 2 Rigid acid Sulfamic acid 75 4.4 7 4 Rigid Ascorbic acid 754.2 4 3 Rigid Glycolic acid 75 4.8 13 10 Partly flexible Cyanoacetic 755.3 28 14 Partly acid flexiblePreparation of an Acidic Particulate Water-Absorbing Polymer (SAP 1)

270 g of acrylic acid were weighed into a glass beaker. 1.155 g ofmethylenebisacrylamide (MBA) crosslinker were then added and themonomers were stirred until completely dissolved. 810 g of distilledwater were weighed into a separate vessel and added to the monomermixture. The solution was stirred to complete the mixture. The aqueoussolution was then kept in a refrigerator for about one hour to cooldown.

Distilled water was used to prepare a 10% sodium persulfate solution andit was added to a cooled polymerization vessel. 0.157 g of2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocur 1173, Ciba,photoinitiator) and 2.736 g of the 10% sodium persulfate solution wereadded as an initiator system. A final mixing step gave a homogeneoussystem which was left alone until it had attained a temperature of 10°C., at which point the polymerization reaction was carried out in thecourse of 12 minutes by irradiating with 20 mWcm⁻² UV energy. This gavea gellike polymer which was comminuted and completely dried at 125° C.The dried polymer obtained was ground and sieved to collect the fractionhaving an average particle size of 150 μm-450 μm.

EXAMPLE 5

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 15 g of a 15% aqueous solution of acommercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker. Thecrosslinkable mixture was foamed in the shearing zone of an Ultraturraxstirrer for 1 minute. Further crosslinkable mixtures were prepared fromthe abovementioned components and the amounts of pentane indicated inTable 7 and foamed by shearing with an Ultraturrax instrument for 1minute.

45 g of SAP 1 (particle size 150-450 μm) were then added and the mixturewas homogenized by further shearing for about 1 minute. The foammixtures thus prepared were then each poured onto a Teflon supportrimmed with aluminum. The foam layer in the molds as 6 mm deep. Themolds containing the foamed crosslinkable mixtures were stored at 70° C.in a drying cabinet overnight. During this period, the polyvinylaminebecame crosslinked and the foam completely dried. The hydrogel foamsobtained in each case were subsequently adjusted to a water content of5%. They had the properties indicated in Table 7. TABLE 7 Pentanecontent Foam FSC CRC FSC CRC AAP of aq. density g/g g/g g/g g/g g/g FSRmix g/cm³ 30 min 30 min 4 h 4 h 0.3 psi g/g/sec  0% 0.66 10.4 6.5 23.518.9 7.2 >0.06 10% 0.20 49.3 20.7 57.4 25.1 30.4 0.95 15% 0.21 49.2 19.156.4 23.9 30.5 0.74 20% 0.19 50.1 19.2 56.3 24.7 27.7 0.66

The test results show that the multicomponent foam system gave the bestperformance when prepared from a starting mixture having a blowing agentcontent of 10% pentane, based on the total amount of the aqueousstarting solution.

EXAMPLE 6

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 15 g of a 15% aqueous solution of acommercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker and 3 gof pentane. The crosslinkable mixture was foamed in the shearing zone ofan Ultraturrax stirrer for 1 minute. The samples of the crosslinkableaqueous mixture were then each admixed with 45 g of SAP 1 having aparticle size distribution indicated in each case in Table 8. Themixture was then stirred for 1 minute. A homogeneous mixture wasobtained.

The crosslinkable foam mixtures thus prepared were then each poured ontoa Teflon support rimmed with aluminum. The foam layer in the molds was36 mm deep. The molds containing the foamed crosslinkable mixtures werestored at 70° C. in a drying cabinet overnight. During this period, thepolyvinylamine became crosslinked and the foam completely dried. Thehydrogel foams obtained in each case were subsequently adjusted to awater content of 5%. They had the properties indicated in Table 7. TABLE8 SAP 1 Foam FSC CRC FSC CRC AAP particle density g/g g/g g/g g/g g/gFSR size g/cm³ 30 min 30 min 4 h 4 h 0.3 psi g/g/sec 150-850 0.2 32.115.1 49.7 24.2 9.3 0.1 μm 150-450 0.2 53.4 23.1 64.0 27.8 9.5 0.65 μm

In general, the particle size section from 150 to 450 μm possesses adistinctly superior absorption profile, due perhaps partly to betterintegration of small particles within the foam system and partly toreduced impairment of fluid transportation within the channel system bylarge swelling SAP 1 particles.

EXAMPLE 7

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylamine(PVAm) having a K value of 90 were added 15 g of a 15% aqueous solutionof a commercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker and 3 gof pentane. The crosslinkable mixture was foamed in the shearing zone ofan Ultraturrax stirrer for 1 minute. Samples of the crosslinkableaqueous mixture were then admixed with SAP 1 having an average particlesize distribution of 150-450 μm in the polyvinylamine:SAP 1 weightratios indicated in each case in Table 9. The mixtures were then eachstirred for 1 minute. Homogeneous mixtures were obtained.

The crosslinkable foam mixtures thus prepared were then each poured ontoa Teflon support rimmed with aluminum. The foam layer in the molds was 6mm deep. The molds containing the foamed crosslinkable mixtures werestored at 70° C. in a drying cabinet overnight. During this period, thepolyvinylamine became crosslinked and the foam completely dried. Thehydrogel foams obtained in each case were subsequently adjusted to awater content of 5%. They had the properties indicated in Table 9. TABLE3 Foam FSC CRC FSC CRC AAP density g/g g/g g/g g/g g/g FSR Sample g/cm³30 min 30 min 4 h 4 h 0.3 psi g/g/sec 30 0.20 41.7 13.5 58.6 19.9 26.30.12 PVAm: 70 SAP 1 40 0.20 49.3 20.7 57.4 25.1 30.4 0.95 PVAm: 60 SAP 150 0.12 37.1 12.0 52.0 19.8 29.4 0.11 PVAm: 50 SAP 1 60 0.12 37.5 11.947.3 18.0 24.8 0.29 PVAm: 40 SAP 1

The best performance of the foams prepared was obtained in the case of acombination of the multicomponent system in a PVAm:SAP 1 ratio of 40:60.

EXAMPLE 8

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylamine(PVAm) having a K value of 90 were added 15 g of a 15% aqueous solutionof a commercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker and 3 gof pentane. The crosslinkable mixture was foamed in the shearing zone ofan Ultraturrax stirrer for 1 minute. The crosslinkable aqueous mixturewas then admixed with 45 g of SAP 1 having an average particle sizedistribution of 150-450 μm and the mixture was then homogenized bytreatment with an Ultraturrax for 1 minute in each case.

Two further crosslinkable foam mixtures of the abovementionedcomposition were prepared and they were then each poured onto a Teflonsupport rimmed with aluminum. The foam layer in the molds was 6 mm deep.The molds containing the foamed crosslinkable mixtures were storedovernight in a drying cabinet at the temperatures indicated in Table 10.During this period, the polyvinylamine became crosslinked and the foamcompletely dried. The hydrogel foams obtained in each case weresubsequently adjusted to a water content of 5%. They had the propertiesindicated in Table 10. TABLE 10 Foam FSC CRC FSC CRC AAP Temper- densityg/g g/g g/g g/g g/g FSR ature g/cm³ 30 min 30 min 4 h 4 h 0.3 psig/g/sec 60° C. 0.21 47.1 21.4 50.2 29.5 28.9 0.70 70° C. 0.21 46.0 16.753.6 21.0 26.5 0.43 90° C. 0.21 41.7 15.2 52.0 19.6 25.0 0.32

Crosslinking and drying temperatures influence the performance todifferent degrees. Whereas the absorption performance in the teabag testis highest after 30 minutes at 60° C., earlier saturation in the 4 hourvalue is achieved compared with higher temperatures. However, retentionvalues and incipient swelling rate are optimal at this temperature.

EXAMPLE 9

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylamine(PVAm) having a K value of 90 were added 15 g of a 15% aqueous solutionof a commercially available surfactant (addition product of 80 mol ofethylene oxide with 1 mol of a C16/C18 alcohol mixture) and 15 g of a 5%aqueous solution of ethylene glycol diglycidyl ether crosslinker and 3 gof pentane. The crosslinkable mixture was foamed in the shearing zone ofan Ultraturrax stirrer for 1 minute. The crosslinkable aqueous mixturewas then admixed with 45 g of SAP 1 having an average particle sizedistribution of 150-450 Mm and the mixture was then homogenized bytreatment with an Ultraturrax for 1 minute in each case.

Further crosslinkable foam mixtures of the abovementioned compositionwere prepared and they were then each poured onto a Teflon supportrimmed with aluminum. The foam layer in the molds was 6 mm deep. Themolds containing the foamed crosslinkable mixtures were stored overnightin a drying cabinet at the temperatures indicated in Tables 11 to 13.During this period, the polyvinylamine became crosslinked and the foamcompletely dried. Thereafter, the foams were stored for 1 hour at thetemperatures indicated in Tables 11 to 13 in order to cure theinterfacial areas between the basic polymer and the acidic polymer. Thehydrogel foams obtained in each case were subsequently adjusted to awater content of 5%. They had the properties indicated in Tables 11 to13. TABLE 11 Drying overnight at 60° C., followed by 1 hour drying atvarious temperatures Drying temper- Foam FSC CRC FSC CRC AAP aturedensity g/g g/g g/g g/g g/g FSR 1 h g/cm³ 30 min 30 min 4 h 4 h 0.3 psig/g/sec  0° C. 0.21 47.1 21.4 50.2 29.5 28.9 0.70 100° C. 0.21 46.4 19.246.6 25.7 27.2 0.50 125° C. 0.21 37.1 16.2 42.3 23.9 26.5 0.25 140° C.0.21 27.3 13.2 35.9 19.5 27.4 0.10

TABLE 12 Drying overnight at 70° C., followed by 1 hour drying atvarious temperatures Drying temper- Foam FSC CRC FSC CRC AAP aturedensity g/g g/g g/g g/g g/g FSR 1 h g/cm³ 30 min 30 min 4 h 4 h 0.3 psig/g/sec  0° C. 0.21 46.0 16.7 53.6 21.0 26.5 0.43 100° C. 0.21 43.2 16.653.2 21.5 25.4 0.46 125° C. 0.21 39.4 14.6 49.7 19.6 26.2 0.26 140° C.0.21 31.5 12.6 40.1 16.4 23.7 0.08

TABLE 13 Drying overnight at 90° C., followed by 1 hour drying atvarious temperatures Drying temper- Foam FSC CRC FSC CRC AAP aturedensity g/g g/g g/g g/g g/g FSR 1 h g/cm³ 30 min 30 min 4 h 4 h 0.3 psig/g/sec  0° C. 0.21 41.7 15.2 52.0 19.6 25.0 0.32 100° C. 0.21 40.1 14.850.8 19.4 24.2 0.24 125° C. 0.21 35.9 15.1 46.5 19.3 22.6 0.14 140° C.0.21 30.1 12.5 40.9 15.8 18.7 0.09

EXAMPLE 10

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 15 g of 5% aqueous solution of acommercially available surfactant (sodium C14-C17 sec-alkylsulfonate,Hostapur® SAS93, from Clariant), 15 g of a 5% aqueous solution ofethylene glycol diglycidyl ether crosslinker and 3 g of pentane.Mixtures containing the amounts of surfactant which are reported inTable 14 were prepared in addition.

The crosslinkable mixture was foamed in the shearing zone of anUltraturrax stirrer for 1 minute and then poured onto a Teflon supportrimmed with aluminum. The mold containing the foamed crosslinkablemixture was stored in a drying cabinet at 70° C. overnight. During thisperiod, the polyvinylamine became crosslinked and the foam completelydried. The hydrogel foam obtained was subsequently adjusted to a watercontent of 5%. The foams had the properties reported in Table 14. Thefoam produced according to Example 2 was stiff, whereas the foamsobtained according to Examples 10-1 to 10-3 were very soft. TABLE 14 AAP@ FSC % Density 0.3 psi (30 min) CRC FSR Example surfactant g/cm³ g/gg/g g/g g/g/sec 10 7.5 0.19 5.2 29.9 10.8 2.88 10 - 1 2.5 0.12 9 35.111.5 7.55 10 - 2 5 0.13 8.4 32.7 11.2 6.64 10 - 3 10 0.13 9.3 30.1 11.33.29

EXAMPLE 11

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 7.5 g of a 30% aqueous solution ofcommercially available surfactant (alkali metal salt of monosulfates ofaddition products of ethylene oxide and propylene oxide with a fattyalcohol, Disponil® FES993 IS ex Cognis), 15 g of a 5% aqueous solutionof ethylene glycol diglycidyl ether crosslinker and 3 g of pentane.Mixtures containing the amounts of surfactant which are reported inTable 15 were prepared in addition.

The crosslinkable mixture was foamed in the shearing zone of anUltraturrax stirrer for 1 minute and then poured onto a Teflon supportrimmed with aluminum. The mold containing the foamed crosslinkablemixture was stored in a drying cabinet at 70° C. overnight. During thisperiod, the polyvinylamine became crosslinked and the foam completelydried. The hydrogel foam obtained was subsequently adjusted to a watercontent of 5%. A foam produced according to Example 2 was stiff, whereasthe foams produced as per Examples 11-1 to 11-3 felt very soft. Furtherproperties of the foams are reported in Table 15. TABLE 15 AAP @ FSC %Density 0.3 psi (30 min) CRC FSR Example surfactant g/cm³ g/g g/g g/gg/g/sec 11 - 1 2.5 0.105 6.7 41.5 12.4 2.4 11 - 2 5 0.106 8.1 40.5 11.73.5 11 - 3 10 0.096 7 38.1 11.7 3.9

EXAMPLE 12

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 7.5 g of a 30% aqueous solution ofcommercially available surfactant (alkali metal salt of monosulfates ofaddition products of ethylene oxide and propylene oxide with a fattyalcohol, Disponil® FES 32 IS ex 40 Cognis), 15 g of a 5% aqueoussolution of ethylene glycol diglycidyl ether crosslinker and 3 g ofpentane. Mixtures containing the amounts of surfactant which arereported in Table 16 were prepared in addition.

The crosslinkable mixture was foamed in the shearing zone of anUltraturrax stirrer for 1 minute and then poured onto a Teflon supportrimmed with aluminum. The mold containing the foamed crosslinkablemixture was stored in a drying cabinet at 70° C. overnight. During thisperiod, the polyvinylamine became crosslinked and the foam completelydried. The hydrogel foam obtained was subsequently adjusted to a watercontent of 5%. A foam produced according to Example 2 was stiff, whereasthe foams produced as per Examples 12-1 to 12-3 felt very soft. Thefoams had the properties reported in Table 16. TABLE 16 AAP @ FSC %Density 0.3 psi (30 min) CRC FSR Example surfactant g/cm³ g/g g/g g/gg/g/sec 12 - 1 2.5 0.094 9.1 42 11.8 3.11 12 - 2 5 0.083 10.4 34.9 11.53.11 12 - 3 10 0.084 10.1 36.5 11.3 2.84

EXAMPLE 13

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 15 g of a 5% aqueous solution of acommercially available surfactant (sodium C14-C17 sec-alkylsulfonate,Hostapur® SAS93, from Clariant) in the amounts reported in Table 17, 15g of a 5% aqueous solution of ethylene glycol diglycidyl ethercrosslinker and 3 g of pentane. Mixtures containing the amounts ofsurfactant which are reported in Table 17 were prepared in addition.

The crosslinkable mixture were then each foamed for 1 minute in theshearing zone of an Ultraturrax stirrer. Samples of the crosslinkableaqueous mixture were then admixed with 45 g of SAP 1. The mixture wasthen stirred for 1 minute. A homogeneous mixture was obtained. The thusproduced crosslinkable mixtures in foam form were then each poured ontoa Teflon support rimmed with aluminum. The mold containing the foamedcrosslinkable mixture was stored in a drying cabinet at 70° C.overnight. During this period, the hydrogel foam obtained was completelydried and subsequently moistened with water to adjust it to a watercontent of 5%. The foams had the properties reported in Table 17. A foamproduced according to Example 6 was stiff, whereas the foams producedaccording to Examples 13-1 and 13-2 felt very soft. TABLE 17 AAP @ FSC %Density 0.3 psi (30 min) CRC FSR Example surfactant g/cm³ g/g g/g g/gg/g/sec 13 - 1 2.5 0.22 8.4 43.6 18.1 1.4 13 - 2 5 0.26 12.4 43.2 17.20.79

EXAMPLE 14

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 7.5 g of a 30% aqueous solution ofcommercially available surfactant (alkali metal salt of monosulfates ofaddition products of ethylene oxide and propylene oxide with a fattyalcohol, Disponil® FES993 IS ex Cognis), 15 g of a 5% aqueous solutionof ethylene glycol diglycidyl ether crosslinker and 3 g of pentane.

The crosslinkable mixture were then each foamed for 1 minute in theshearing zone of an Ultraturrax stirrer. The foamed sample was thenadmixed with 45 g of SAP 1, stirred for 1 minute and then poured onto aTeflon support rimmed with aluminum. The mold containing the foamedcrosslinkable mixture was stored in a drying cabinet at 70° C.overnight. During this period, the hydrogel foam obtained was completelydried and subsequently moistened with water to adjust it to a watercontent of 5%. A foam produced according to Example 6 was stiff, whereasthe foams produced as per Example 14-1 felt very soft. The foam had theproperties reported in Table 18. TABLE 18 AAP @ FSC % Density 0.3 psi(30 min) CRC FSR Example surfactant g/cm³ g/g g/g g/g g/g/sec 14 - 1 2.50.16 9.6 44.1 19.5 1.3

EXAMPLE 15

To 300 g of a 10% ultrafiltered aqueous solution of polyvinylaminehaving a K value of 90 were added 7.5 g of a 30% aqueous solution ofcommercially available surfactant (alkali metal salt of monosulfates ofaddition products of ethylene oxide and propylene oxide with a fattyalcohol, Disponil® FES 32 IS ex Cognis), 15 g of a 5% aqueous solutionof ethylene glycol diglycidyl ether crosslinker and 3 g of pentane. Amixture containing the amounts of surfactant which are reported in Table19 was prepared in addition.

The crosslinkable mixtures were then each foamed for 1 minute in theshearing zone of an Ultraturrax stirrer. The foamed sample was thenadmixed with 45 g of SAP 1, stirred for 1 minute and then poured onto aTeflon support rimmed with aluminum. The mold containing the foamedcrosslinkable mixture was stored in a drying cabinet at 70° C.overnight. During this period, the hydrogel foam obtained was completelydried. The water content of the hydrogel was thereafter adjusted to 5%.The foam was very soft and had the properties reported in Table 19.TABLE 19 AAP @ FSC % Density 0.3 psi (30 min) CRC FSR Example surfactantg/cm³ g/g g/g g/g g/g/sec 15 - 1 2.5 0.19 7.7 42.8 19.6 0.67

1. A foam comprising a water-absorbing basic polymer obtainable by (I)foaming a crosslinkable aqueous mixture including (a) at least one basicpolymer whose basic groups have optionally been neutralized, (b) atleast one crosslinker, (c) at least one surfactant, (d) optionally atleast one solubilizer, (e) optionally thickeners, foam stabilizers,fillers, fibers, cell nucleators, and mixtures thereof and (f)optionally particulate water-absorbing acidic polymers, by dissolving agas which is inert toward free radicals in the crosslinkable aqueousmixture under a pressure from 2 to 400 bar and subsequentlydecompressing the crosslinkable aqueous mixture to atmospheric or bydispersing fine bubbles of a gas which is inert toward free radicals,and (II) crosslinking the foamed mixture to form a hydrogel foam and ifapplicable adjusting a water content of the polymer foam to 1-60% byweight.
 2. The foam of claim 1, wherein the basic polymer comprisespolymers containing vinylamine units, polymers containing vinylguanidineunits, polymers containing dialkylaminoalkyl(meth)acrylamide units,polyethyleneimines, ethylenimine-grafted polyamidoamines and,polydiallyldimethylammonium chlorides, or a mixture thereof.
 3. The foamof claim 1 wherein the basic polymer comprises polymers containingvinylamine units, polyvinylguanidines, polyethyleneimines, or a mixturethereof.
 4. The foam of claim 1 wherein the basic polymer comprisespolyvinylamines and/or up to 10-100% hydrolyzed poly-N-vinylformamides.5. The foam of claim 1, whose surface has been postcrosslinked.
 6. Afoam of claim 1 wherein the water-absorbing basic polymers additionallyinclude finely divided water-absorbing acidic polymers, the polymermixture including from 10 to 90% by weight of water-absorbing acidicpolymers.
 7. A foam of claim 1 wherein the water-absorbing acidicpolymers are crosslinked acrylic acids having a particle diameter from10 to 2000 μm.
 8. A process for producing foams comprising awater-absorbing basic polymer, which comprises (I) foaming acrosslinkable aqueous mixture including (a) at least one basic polymerwhose basic groups have optionally been neutralized, (b) at least onecrosslinker, (c) at least one surfactant, (d) optionally at least onesolubilizer, (e) optionally thickeners, foam stabilizers, fillers,fibers, cell nucleators, and mixtures thereof, and (f) optionallyparticulate water-absorbing acidic polymers. by dissolving a gas whichis inert toward free radicals in the crosslinkable aqueous mixture undera pressure from 2 to 400 bar and subsequently decompressing thecrosslinkable aqueous mixture to atmospheric or by dispersing finebubbles of a gas which is inert toward free radicals, and (II)crosslinking the foamed mixture to form a hydrogel foam and ifapplicable adjusting a water content of the polymer foam to 1-60% byweight.
 9. The process of claim 8 wherein from 0.05 to 20 parts byweight of an acidic water-absorbing polymer having a degree ofneutralization from 0 to 75 mol % are used per part by weight of a basicpolymer.
 10. The process of claim 8 wherein the crosslinkable aqueousmixture includes from 0.1 to 30% by weight of a hydrocarbon. 11.(Cancelled)
 12. A hygiene article for absorbing body fluids comprising afoam of claim
 1. 13. A dressing material for covering wounds comprisinga foam of claim
 1. 14. A sealing material comprising a foam of claim 1.15. A packaging material comprising a foam of claim
 1. 16. A soilimprover comprising a foam of claim
 1. 17. A soil substitute comprisinga foam of claim
 1. 18. A method of dewatering sludges comprisingcontacting the sludge with a foam of claim
 1. 19. A method of absorbingaqueous acidic wastes comprising contacting the acidic waste with a foamof claim
 1. 20. A method of thickening waterborne paints comprisingcontacting a paint with a foam of claim
 1. 21. A method of dewatering anoil or a hydrocarbon comprising contacting the oil or hydrocarbon with afoam of claim 1.